1. Field of the Invention
This invention relates to an alignment technology for aligning two substrates by arranging them opposite to each other and a transfer technology for transferring a pattern or a thin film as a transfer material carried on one substrate to a predetermined position of the other substrate.
2. Description of the Related Art
As an alignment technology for superposing two substrates, for example, a technology disclosed in patent literature JP2004-151653A is known. In this technology, an alignment mark is formed on a surface of each of the two substrates to be bonded, and an alignment process is performed based on images imaged by an imager (e.g. CCD camera). Specifically, by arranging the both substrates such that the alignment mark formed surfaces face each other, a distance between the alignment marks is set to be equal to or shorter than the depth of field of the imager, whereby imaging is performed in a state where the both alignment marks are in focus. Relative positions between the substrates are adjusted based on a positional relationship of the both alignment marks detected from the imaged image.
Such an alignment technology is also applicable to a pattern forming method for forming a predetermined pattern on another substrate by transferring a pattern carried on one substrate to the other substrate. That is, the pattern can be formed at an appropriate position on the other substrate by aligning the one substrate carrying the pattern and the other substrate, to which the pattern is to be transferred, with high accuracy.
Further improvement in alignment accuracy is desired in such a technology. It is thought as one effective method to increase the magnification of an image to be imaged. However, since a depth of field is generally reduced if the magnification of an imaging optical system of an imager is increased, both substrates need to be brought closer to each other in the above conventional technology based on the premise that imaging is performed in a state where the both alignment marks are in focus. However, there is an appropriate range for the distance between the substrates in consideration of dimensional variations, deflection and the like of the substrates and a mechanism for holding them. Thus, the both alignment marks may not be able to be arranged within the range of the depth of field shortened by increasing the magnification. The above conventional technology cannot deal with such a case.
It is thought as another method to individually image two alignment marks by successively focusing the imager on these alignment marks. However, the above conventional technology cannot deal with such a case. Further, a detection error resulting from a variation of an optical axis by a focusing operation may rather reduce alignment accuracy.